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{
   my $foo = {}; # ref count 1
...
# it refers to it self, its self-referential,  a circular reference
# for memory held by $foo to be released after $foo goes out of scope
# $foo->{foo} has to be weakened, the reference count decreased

sub once {
  my ($self, $name, $cb) = @_;
...

  return $wrapper;
}